The present invention relates to a document feeder having a separating device for separating and feeding a lowest sheet of stacked sheets or documents by rotatably contacting the lowest sheet, which is useful for automatically feeding the sheets or documents in a copying machine or facsimile machine.
As a prior art, there is an automatic document feeder K as a sheet feeding device including a sheet feeding roller for separating and feeding a lowest sheet of stacked sheets or documents mounted on a sheet feeding tray by rotatably contacting the lowest sheet, as shown in FIG. 11.
In the automatic document feeder K, the stacked documents or sheets are separated, and are fed one by one onto a platen P of a copier C. The automatic document feeder K is formed as follows.
In FIG. 11, a separating portion 125 for separating the stacked documents or sheets one by one and feeding thereof includes a sheet feeding roller 126 for feeding the separated sheet in a forward direction, and a separating member for preventing the document after the second sheet from being fed in the forward direction, which contacts the sheet feeding roller 126. The separating member is formed of a separating pad 127, and is made of a material having at least a large friction coefficient relative to the sheet so that a frictional force greater than that between the sheets acts between the separating pad and a document or sheet. The separating pad 127 is urged toward a side of the sheet feeding roller 126 by a holding spring 129 provided between a pad holder 128 and a spring holder 130.
Sheets or documents stacked on a sheet feeding tray 111 are fed as a batch of plural documents or sheets from a lower side by a pick-up roller 121 for constituting a feeding device together with a guide plate 113; the batch of sheets are passed through a front sweeping plate 114 and are guided to a separating portion 125 while forward ends thereof are being moved or separated diagonally; and only the lowest sheet is separated by the sheet feeding roller 126 of the separating portion 125 and the separating pad 127, and is fed to a reverse space portion 133. In the separating operation, when the batch of the sheets is fed by the pick-up roller 121, the batch of sheets is pressed from an upper side toward a down side with a weight plate 123 by means of a driving device, such as a solenoid which is not shown, so that a frictional force between the pick-up roller 121 and the batch of sheets is increased to thereby obtain a force for carrying the sheets to the sheet feeding roller 126.
A pressing action of the weight plate 123 continues until the batch of the sheets is nipped by the sheet feeding roller 126 and the separating pad 127, and thereafter it is released. The pressing action of the weight plate 123 increases a frictional force between the lowest sheet and the pick-up roller 121 as well as frictional forces among the sheets, which becomes an obstacle to the separating action by the separating portion 125. Thus, it is necessary that once the lowest sheet is nipped by the separating portion 125, the pressing action is released.
However, in the above-described conventional document feeder, since a relatively large number of documents or sheets enters into a space between the separating roller 126 and the separating pad 127 of the separating portion 125, there have been the following problems.
1. When the lower sheets of a batch of the stacked documents or sheets are subjected to a large friction force between the sheets due to a weight of the stacked documents or sheets, the lowest sheet and the second sheet thereon are forced to slide with a large friction force at the separating portion 125. Therefore, color of the characters or ink on a surface of the lowest sheet is transferred to a back surface of the second sheet thereon, so that the back surface of the second sheet is smudged, or the surface of the lowest sheet is smudged by being rubbed with the back surface of the second sheet. This phenomenon causes a problem in case manuscripts written by a pencil or a ball-point pen are handled in an automatic document feeder of a copying machine and facsimile machine.
2. Also, in case friction forces among lower sheets are large due to a weight of a batch of the stacked documents or sheets, a plurality of sheets or documents including the lowest sheet and other sheets piled thereon is apt to be fed at the same time by the pick-up roller 121. These sheets may enter, at the same time, a gate by the front sweeping plate 114 provided in front of the sheet feeding roller, or a space between the sheet feeding roller and the separating pad 127 which slidably contacts the sheet feeding roller for preventing sheets above the lowest sheet from being advanced, to thereby cause, what is called a wedge phenomenon, which results in a mis-feed or a double feed.
The present invention has been made to obviate the above problems, and an object of the invention is to provide a document feeder, wherein a back surface of a sheet or document is not smudged at a time of separation.
Another object of the invention is to provide a document feeder as stated above, wherein the mis-feed and double feed due to the wedge phenomenon does not occur, and the sheets or documents can be smoothly separated.
Further objects and advantages of the invention will be apparent from the following description of the invention.